Noncorrosive media for heating transfer



Feb. 5, 1946. c; G. GERHOLD NONCORROSIVB EDIA FOR HEATING TRANSFER FiledSept. 30, 1944 Patented Feb. 1946 NONCOBROSIVE MEDIA FOR HEATINGTRANSFER Clarence G. Gerhold, Chicago, IIL, assigner to Universal i]Products Company, Chicago, Ill., a corporation oi' Delaware ApplicationSeptember 30, 1944, Serial No. 556,621

22 Claims.

This invention relates to improvements in the manufacture of fused saltmixtures and to improved methods and means for handling such mixtures toprevent corrosion of the equipment with which they are brought incontact.

Fused metal salts, particularly mixed metal halides, are well suited foruse as heat conductive media because of their low melting points, lowvapor pressures, extreme stabilities, high heat transfer coeflicientsand high heat capacity per unit volume. A great deal of diiliculty hasbeen experienced, however, in using such mixtures due to corrosion ofthe equipment with which they are in contact. The present invention isdirected towards means of eliminating this corrosion.

The corrosion in such systems falls into the two general types, namelyreplacement corrosion and electrolytic corrosion.

Replacement corrosion occurs when fused metal salts'are employed havingcations whose position in the electromotive series is lower than that ofthe metal of the walls of the vessel. In these cases a substitutionoccurs in which the metal from the walls displaces the cations in thesalt mixture lower in the electromotive series 25 thereby effectingcorrosion of the walls. For example, if one of the components of thesalt mixture were cuprous chloride the salt would attack a steel vesselresulting in deposition of the copper and the substitution of iron ionsin the salts in place of the copper ions. This replacement reaction maybe set up as follows:

If the cations are higher in the electromotive series than the metal ofthe walls of the vessel, for example iron, their replacement reactionFe+M+++ Fe++++M cannot proceed unless energy in the form ofelectromotive force is applied. The magnitude of the necessary EMFdecreases with the proximity of the cations to iron in the electromotiveseries. This latter type of corrosion is usually spoken of aselectrolytic corrosion and is the more viscous of the two types inrespect to the deterioration of the walls of the chamber. It has 'beenpreviously proposed to add iron salts to the salt mixtures to preventcorrosion of the vessel, the thought being that the presence of ironcations in the salt mixture would in accordance with the law of massaction suppress the tendency of the iron in the vessel walls to go intosolution. However, I have discovered that the rate of corrosion of ironvessels which are contacted with fused salt mixtures is directly ture.Although the exact reason for this is not known it can probably beexplained as follows: As previously stated, when using salts havingmetal cations higher in the electromotive series than the metals of thewalls of a vessel as heat carrying media, it is necessary that someelectromotive force be applied before any displacement of the metalcations of the salt mixture by the metal of the vessel walls occurs. Itis also true that the potential necessary decreases as the respectivepositions of the metals in the electromotive series approach oneanother. Therefore, presence of cations which are similar to the metal.of the wall provides a setup in which any slight potential would besuiiicient to effect displacement and the attending corrosion. Further,with increases in the concentration of the iron cations, excessiveamounts of corrosion will result as a result of the increase in currentdensity. I have discovered that by maintaining the iron cationconcentration in the fused salt mixture below about 300 to 400 parts permillion and preferably below about 200 parts per million, the rate ofcorrosion is very low and the equipment can be used for long extendedperiods of operation without replacement. For example, in a casein whichan iron vessel contains a bath of very pure sodium aluminum chloridemixture, that is a mixture in 0 which the cations in the bath are allconsiderably higher in the electromotive series than the walls of thevessel, the ow of current can only occur by corrosion of the iron andcorresponding depositions of aluminum or sodium. Since this 5 requires adefinite and fairly high potential no corrosion would be encountered insuch a system until this potential were reached, Theoretically speaking,therefore, if a very pure salt were maintained no diilculties withcorrosion would result. However, as a practical measure it is impossiblein the ordinary operation to maintain the degree of purity necessary toavoid corrosion.

In equipment of commercial size ordinarily small amounts of iron areintroduced into the salt mixture by scaling or perhaps by replacementcorrosion which oecurs if small amounts of water are present in thesystem forming HCl. Since hydrogen can be replaced vby iron ions smallamounts of iron chloride are formed in the salt mixture. As the amountof iron ions increases due to the accumulation in the salt mixture apoint is reached where a very low electromotive force will causesubstantial electrolytic corrosion.

In vessels of commercial size this driving potenrelated to the presenceof iron cations in the mixtial may be set up due to temperaturegradients ,ws-arisenor due to contacts with dissimilar metal. As aresult of the presence of the iron cations in the salt mixtures theselow potentials obtained by temperature differences in the vessel whichwould ordinarily be insufficient to cause excessive electrolyticcorrosion may because of the low potential now necessary effectcorrosion at a substantial rate and subsequently result in failure ofthe vessel.

I have discovered a method for effecting continuous utilization of heatcarrying media by maintaining the purity of said media at a level suchthat no accumulation of iron cations occurs thereby substantiallypreventing the previously described electrolytic corrosion.

I have found that if the salt solutions are kept in contact with a metalwhich is higher in the electromotive series than any of the componentsof the vessel walls and which preferably corresponds to the cation inthe salt mixture lowest in the electromotive series, the impurities inthe salt solution are removed thereby preventing the accumulation ofthese impurities in the salt solution and eliminating or substantiallyreducing any electrolytic corrosion. The metal introduced into the saltsolution may be in a finely divided state such as metal powder or whendesired may be in the form of a plate electrode.. In the latter case itis preferable that the electrode is electrically insulated from thewalls of the containing vessel to prevent the metal from going intosolutions and plating on the walls of said vessel.

Salt mixtures which may be employed in the process of the presentinvention comprise such metal salt mixtures as potassiumA zinc chloride,lithium aluminum chloride, sodium aluminum chloride, mixtures of sodiumaluminum chloride and potassium aluminum chloride, sodium nitrate andsodium nitrite and similar materials. The preferred salt mixtures arethose comprising mixtures of the halides of the alkali metals andaluminum halides and particularly the chlorides such as sodium chlorideand aluminum chloride.

In selecting the metals to be employed to replace the impuritiesaccumulating in the salt mix- 45 ture it is important that the metalchosen will not upon replacement change the composition of the saltmixture appreciably in order for the characteristics of the saltmixture, such as heat capacity, etc., to remain substantially constant 5throughout the operation. Further, it should be noted that in someinstances alloys of metals may be used instead of a single metal toeffect the removal of impurities.

In a cyclic system in which the fused metal 55 salts are passed througha heating means and then inA indirect heat exchange with materialsundergoing reaction such as hydrocarbon conversion reactions, forexample, dehydrogenation,

catalytic cracking, catalytic reforming, catalytic 00 aromatization,etc., the process of the present invention may be used to greatadvantage. The

I advantages will be more clearly set forth hereinafter in thespecification and description of a catalytic dehydrogenation process toproduce butadiene utilizing a fused salt mixture as a heat carryingmedia.

In one embodiment'my invention comprises a' system for effecting heattransfer employing a fused salt mixture in indirect heat exchange withthe materials to be heated, said mixture being characterizedin that thecations areof metals which are higher in the electromotive series thanthe metal of the walls of the vessel, which comprises passing said heatcarrying medium through a heating zone and subsequently through a zonein which a portion of the heat contained in said medium is removed byindirect heat exchange and.

stituents of said metal salt mixture and preferably with a metalcorresponding to cations of the metal lowest in the electromotiveseries.

The following is a description of a diagrammatic sketch of a process forthe conversion of normal butane to butadiene employing a heat carryingmedium consisting of a mixture of sodium and aluminum chlorides.Although the description is limited to a butadiene process, it is notintended that the general broad scope of the invention be so limitedsince the invention is applicable to various processes for theconversion of organic compounds, particularly hydrocarbons employingsome means of transferring heat from a furnace or other heating means toa reaction zone.

For simplification, the diagrammatic sketch includes only elementsnecessary for the explanation of the invention. In the presentdescription, a two-reactor system is employed, the catalyst in one ofthe reactors being utilized for the conversion operation, while thecatalyst ,in the other reactor is simultaneously undergoing regenerationby the oxidation of the carbonaceous deposits laid down upon thecatalyst during aprevious conversion operation. By alternatelycontacting the catalyst with the reactants and regenerating medium, asubstantially constant over-all operation is obtained.

Referring to the drawing, normal butane is introduced in the systemthrough line I containing valve 2, commingled with recycled normalbutane and butylenes obtained as hereinafter set 40 forth, andintroduced into pump 3 which discharges through line 4 containing valve5 into heating coil 6 disposed within furnace 1 wherein the combinedfeed is raised to a temperature sufficient to compensate for heat lossesby radiation, convection, etc. and still maintain the desiredtemperature in catalyst tube I2. The heated combined feed leaves furnace1 through line 8 and is directed to line 9 containing valve l0 throughline II into catalyst tube I2 wherein it is contacted with a catalystcapable of eiecting dehydrogenation of the normalbutane-butylene'mixture to a substantial yield of butadiene.

Any suitable catalyst capable of accelerating dehydrogenation may beemployed in reactor tube I2 such as, for example, the oxides of thelefthand columns of Groups V and VI of the Periodic Table supported onrefractory materials such as silica, alumina, particularly an aluminahaving the characteristics of Activated Alumina of commerce, magnesia,titania, etc. The preferred catalyst is one comprising chromium oxidesupported on Activated Alumina of commerce. 'Ihe temperatures employedin catalyst tube I2 may be within the range of about 950 to 1200 F. andwill ordinarily be in the range of about 1000" to 1150 for theconversion of a butane-butylene mixture to butadiene. However, due tothe endothermicity of the reaction, it is necessary to provide somemeans of maintaining a substantially constant temperature in thereaction tube. If this is not done, a considerable temperaturedifferential will result between the inlet temperature and outlettemperature, thereby affecting the conversion and eilils so operatedthat it ciency of the operation. 'I'he temperature is maintained in thissystem by indirectheat exchange with a solution of fused salts passedthrough jacket I3 in contact with the outer surface of tube I2. Themolten salt is passed through line 2'I containing valve 28 from salttank 26 and is pumped by pump 29 through line 30 containing valve 3Ithrough switch valve 32 which can direct the flow into the annular spacebetween tube I2 and jacket I3 or tube 2| and jacket 22 through lines 33or G9, respectively.

In the present description, since the catalyst in tube I2 is beingemployed for the endcthermic dehydrogenation reaction While the catalystin tube 2l is being regenerated, the flow of heated salt is directedthrough switch valve 32 through line 33 into annular space between tubeI2 and jacket I3. The salt passes upwardly through the annular space incontact with the outer surface of reactor tube I2 and leaves the heatingzone through line 49, through switch valve 50 which directs the iiowthrough line 5I through heating coil 52 disposed within furnace 53wherein the salt is raised to the desired temperature necessary tocompensate f r all heat losses in the system and still maintain thedesired temperature in the heating jacket I3. The salt leaves furnace 53through line 54, valve 55 and is directed into salt tank 26 wherein itis contacted with bar aluminum which reduces any iron ions present inthe system because of the presence of small amounts of impurities suchas HC1, and said iron is deposited as a sludge in the bottom of salttank 26, The sludge is periodically removed from tank 28 through line 58containing valve 5T. Instead of using bars of aluminum in the salt tank,finely divided aliuminum may be introduced into the circulatory .systemand thesludge accumulated in a trap of some sort and removed from thesystem by this method.

As previously stated in the butane dehydrogenation process, at least tworeactors are employed with the ow of hydrocarbon and regenerating gasbeing alternated between the reactors to give a more continuousoperation. In this description, it has beenassumed that the catalyst inreactor 2I is undergoing regeneration. Since this reaction is highlyexothermic, it is necessary to provide some means for removing thecomplished by stream of fused salt in indirect heat exchange with thecatalyst reactor during the regeneration operation to remove the excessheat. The system employed is substantially the same as that describedpreviously for the addition of heat to reactor I2 with the exceptionthat furnace 53 is replaced by cooler 82. The flow of the system is asfollows: fused salt is pumped from tank 34 through suction line 35containing valve 36 into pump 31 .which discharges through line 38,valve 39 and switch valve 32 which directs the iiow through the annularspace between jacket 22 and reaction tube 2|. The cool salt passesupwardly through said annular space, thereby absorbing excess heatduring its passage and is removed through line l0 into switch valve 50which directs the flow through line $0, valve BI, into cooler 62 whereinits temperature is reduced sufciently to facilitate its use as a coolingmedium. The cooled salt still in liquid form is passed -covery systemwherein through line 83 containing valve 34 into tank 34 wherein it iscontacted with bar aluminum to reduce the iron cations present, therebyforming a sludge and maintaining a salt in a substantially pure state.The sludge is removed through line 58 containing valve 59.

The regenerating gas usually comprising a combustion gas containingregulated amounts of oxygen is introduced into the system through line`II and may be directed through either lines 46 or 4l containing valves45 and 48, respectively, depending upon which reactor tube is beingregenerated. As previously stated, while the catalyst in reactor tube 2Iis in the process of regeneration the ilow 1s through line 4I containingvalve '48 into line 20 through the catalyst bed in reactor 2l and thecarbonaceous material from the catalyst in tube 2I removed by thepassage of the regenerating gas through the catalyst bed. Ordinarily, 1tis necessary that the regenerating gas be heated to a temperature ofabout 600 to 800 F. to initiate combustion. However, to simplify theexplanation of the drawing, no heating means is shown. The regeneratinggases are removed from tube 2I through line -23 and directed throughline 43 containing valve 44 into line 42 and may be vented to theatmosphere or may be recycled through the reactor after introduction ofadditional oxygen-containing gases and the excess vented to maintain aconstant pressure throughout the regeneration cycle.

The products of the dehydrogenation reaction are removed through lineI`I into a suitable rethe butadiene is separated from the remaininghydrocarbons by any of the well known means such as azeotropicdistillation, formation of chemical compounds, etc. and the remainingmaterial comprising normal butane and butylenes introduced through line65 and valve S8 into line I wherein it is commingled with the freshbutane charge to form the com` bined feed to the operation.

In the above description of the drawing, the catalyst reactors have beenrepresented as single tubes disposed in the heating jacket. However,`for commercial operation, a number of tubes in two or more banksconnected by a common inlet manifold are disposed withina common heatingor cooling jacket with the conversion or regeneration reaction beingconducted simultaneously in all the tubes ineither bank.

The following example presents a comparison of the data obtained in acirculatory salt system in which a fused salt of sodium and aluminumchloride was employed as a heat carrying medium. These data wereobtained by circulating a salt mixture through stainless steel equipmentfor a period of 132 hours.

Rate of penetration No alumi` Aluminum p'gegt present Temp.. oF.

ture of 1100-1150 F. tor a period of 49 days. The following data wereobtained:

Rate of Mean penetration Weight diameter Days on test Inches Inches/yearone of said cations.

2. In a heat transfer process wherein a molten metal salt mixture ispassed through a confined least one metal corresponding to one of saidcations.

than the metals of the Walls of said confined ture with a metalcorresponding to the cation constituent of said mixture which is lowestin the electromotive series.

4. The method of claim 2 further characterized 'in that said moltenmetal salt mixture comprises a molten mixture of metal halides.

5. The method of claim 2 further characterized in that said molten metalsalt mixture comprises a molten mixture of metal chlorides.

7. In a heat transfer process wherein a molten sodium aluminum chloridemixture is passed through a confined zone in indirect heat exchange.relationship with another fluid, the walls with aluminum.

8. In a heat transfer process wherein a molten sodium aluminum chloridemixture is passed through a confined zone in indirect heat exlower inelectromotive series than aluminum, the method which comprises passingsaid molten metal salt mixture together with finely divided aluminumthrough a heating zone and subsequently through a Zone in which aportion of the heat contained in said molten salt mixture is removed byindirect heat exchange.

9. In a heat transfer process wherein a molten metal salt mixture ispassed through a confined zone having walls composed predominantly ofpassing said heat carrying medium through a heating zone andsubsequently through a zone in which a portion of the heat contained insaid medium is removed by indirect heat exchange and maintaining theiron cation concentration in said molten salt mixture below about 400parts per million by contacting said molten salt mixture with a metalcorresponding to one of said cations.

10. The process of claim 8 further characterized in that said moltenmetal salt mixture comprises a mixture of metal chlorides.

11. In a heat transfer process wherein a molten sodium aluminum chloridemixture is passed Athrough a confined zone having walls composed 12. Ina heat transfer process wherein a molten sodium aluminum chloridemixture is passed through' a zone in which a, portion of the heatcontained in said mixture is removed by indirect heat exchange andmaintaining the iron cation concentration in said molten salt mixturebelow about 400 parts per million by passing finely divided aluminumthrough said zones in admixture with the molten salt mixture.

13. A method for preventing corrosion of the walls of a retaining zone,said walls being composed predominantly of iron and contacted with amolten metal salt mixture characterized in that the cations of the saltsthereof are of metals higher in the electromotive series than iron.which comprises maintaining the iron cation concentration in said moltensalt mixture below about 400 parts by weight per million by contactingsaid mixture with a metal corresponding to one o! said cations.

14. The method for preventing corrosion of claim 13 furthercharacterized in that said molen metal salt mixture comprises a mixtureof metal halides.

15. The method for preventing corrosion o! claim 13 furthercharacterized in that said molten metal salt mixture comprises a mixtureof aluminum chloride and at least one alkali metal chloride.

16. The method for preventing corrosion of claim 13 furthercharacterized in that said molten metal salt mixture comprises sodiumaluminum chloride.

17. In a heat transfer process wherein a molten mixture of metal saltsis employed as an indirect heat exchange medium, the method whichcomprises maintaining a source of supply of said molten mixture of metalsalts, cyclically circulating molten salt mixture between said source ofsupply and an indirect heat exchange zone defined by walls of a metallower in the electromotive series than the metals of said salts, wherebycations'of the metal of said walls tend to accumulate in the circulatingmolten mixture, contacting the circulating salt mixture with a metalhigher in the electromotive series than the metal of said walls toreduce said cations, and separating the thus reduced cations from thecirculating molten salt mixture.

18. The method as defined in claim 17 further characterized in that saidmetal higher in the electromotive series than the metal of said wallscorresponds to the metal of one of said salts.

19. The method as defined in claim 17 further characterized in that saidmetal higher in the electromotive series than the metal of said wallscorresponds to the metal of said salts which is lowest in theelectromotive series.

20. A heat transfer method which comprises cyclically circulating moltensodium aluminum chloride through and between a heating zone and anindirect heat exchange zone defined by walls composed predominantly ofiron, whereby iron cations tend to accumulate in the circulating moltenchloride, contacting the circulating molten chloride with metallicaluminum to reduce said cations and thereby deposit iron from the moltenchloride, and separating the deposited iron from the circulating moltenchloride.

21. A heat transfer method which comprises cyclically circulating moltensodium aluminum chloride through and between a heating zone and anindirect heat exchange zone defined by walls composed predominantly ofiron, whereby iron cations tend to accumulate in the circulating moltenchloride, contacting the circulating molten chloride with metallicaluminum in a separating zone between the heating zone and the heat exchange zone te reduce said cations and thereby deposit iron in theseparating zone, and withdrawing the deposited iron from the separatingzone.

22. I'he method as defined in claim 20 further characterized in thatsaid metallic aluminum is circulated in finely divided form through saidzones in admixture with the molten sodium aluminum chloride. l CLARENCEG. GERHOLD.

